Method and apparatus for machining a workpiece ultrasonically

Abstract

In uninterrupted and neat machining of a workpiece or material, particularly extremely hard material, with an ultrosonic tool is obtained by keeping constant or substantially constant the contact pressure between the tool and the worked surface. This is done, for example, by feeding the tool by means of a spindle sleeve which is mounted for displacement in a feed head. The other end of the spindle sleeve, remote from the tool, is designed as the piston of a feed mechanism. By keeping the pressure P1 in the cylinder space above piston, or the volume of this cylinder space, constant, the contact pressure of the tool remains constant as desired. Since the piston displaced during the matching operation, this displacement must be determined, by means of a displacement pickup, for example. Through a very sensitive contact pressure control on a feed head of the working machine, to which the cylinder of the feed mechanism is secured, can be moved to keep the volume of or pressure in the cylinder space constant.

What is claimed is: 
     
       1. A method of machining a workpiece by means of ultrasound using an ultrasonic machine having a tool head for movement toward and away from the workpiece on a feed path, and an ultrasonic machine tool mounted for movement on the tool head and with respect to the tool head toward and away from the workpiece on the feed path, comprising feeding the tool head on the feed path toward the workpiece to establish a contact pressure between the machine tool and the workpiece which is maintained substantially constant, measuring an instantaneous absolute displacement of the machine tool on the tool head in the feed path and from a zero reference point as the machine tool penetrates the workpiece to form a displacement signal corresponding to the instantaneous absolute displacement, using the displacement signal to feed the tool head so as to maintain the substantially constant contact pressure between the machine tool and the workpiece, and repositioning the machine tool to its zero reference point on the tool head by moving the machine tool along the feed path. 
     
     
       2. A method according to claim 1, including using a displacement pickup to measure the instantaneous absolute displacement and form the displacement signal, using a pressure fluid feed mechanism for repositioning the machine tool on the tool head, using a controller for feeding the tool head, the controller being operatively connected to the pressure fluid feed mechanism for supplying pressure to the pressure fluid feed mechanism, and converting the displacement signal into a pressure for the controller to apply to the pressure fluid feed mechanism when a contact pressure between the machine tool and the workpiece drops so as to maintain the contact pressure substantially constant. 
     
     
       3. A method according to claim 2, including comparing the displacement signal with a desired displacement for the machine tool to obtain a differential value and applying the differential value to the controller as the pressure for the pressure fluid feed mechanism, the pressure fluid feed mechanism comprising a cylinder connected to the tool head and a piston movable in the cylinder connected to the machine tool, the piston dividing the cylinder into two chambers, the pressure from the controller being applied to one of the chambers for establishing a desired volume of that chamber. 
     
     
       4. A method according to claim 3, wherein variations in the contact pressure between the machine tool and the workpiece have a mechanical natural resonance which is characteristic of the workpiece, amplifying and shaping the differential signal to form an adjusted differential signal which is optimized for the mechanical natural resonance of the variations in contact pressure. 
     
     
       5. A method according to claim 2, wherein the pressure fluid feed mechanism comprises a cylinder connected to the tool head and a piston connected to the machine tool and movable in the cylinder, the pressure which is converted from the displacement signal being selected as a desired operating pressure for supply to the cylinder for displacing the piston. 
     
     
       6. A method according to claim 1, wherein the tool head has a stator for producing a magnetic field, the machine tool having a translator connected thereto and movably mounted on the tool head to move due to the magnetic field, the stator and its interaction with the translator repositioning the machine tool on the tool head, using a displacement pickup for measuring the instantaneous absolute displacement and for generating the displacement signal, and using a servo-controller operatively connected to the tool head for feeding the tool head, the controller receiving the displacement signal for moving the tool head. 
     
     
       7. A device for machining a workpiece by means of ultrasound, comprising a tool head, a spindle sleeve mounted for displacement on the tool head in a direction toward and away from the workpiece on a feed path, an ultrasonic machine tool connected to said spindle sleeve for movement on the feed path, a displacement pickup engaged between said tool head and said spindle sleeve for measuring relative displacement between said spindle sleeve and said tool head on the feed path, first feed means operatively connected to said tool head and being connected to said displacement pickup for receiving signals from said displacement pickup, said first feed means being operatively connected for moving said machine tool toward the workpiece at a rate to maintain a substantially constant contact pressure between the machine tool and the workpiece, and second feed means connected between said tool head and said machine tool for repositioning said machine tool to a reference zero position on said tool head. 
     
     
       8. A device according to claim 7, wherein said first feed means comprises a fluid pressure cylinder connected to said tool head, a piston movable in said cylinder and having a rod portion extending through said tool head and carrying said spindle sleeve with said feed tool, an ultrasonic converter carried by said spindle sleeve, said displacement pickup being engaged with said ultrasonic converter, a control connected to said pick-up and being responsive thereto to regulate a pressure in said cylinder on each side of said piston and an ultrasonic transformer carried by said ultrasonic converter and carrying the machine tool. 
     
     
       9. A device according to claim 8, wherein said displacement pick-up comprises an inductive pick-up which is secured to said tool head and includes a contact pin applied against said ultrasonic converter. 
     
     
       10. A device according to claim 9, wherein said cylinder comprises a double-acting cylinder, said piston dividing said cylinder into two opposite spaces, one on each side of said piston. 
     
     
       11. A device according to claim 8, including an oscillator-demodulator unit connected to said pick-up for converting an absolute displacement of said machine tool with respect to said tool head into a displacement signal, a comparator connected to said oscillator-demodulator unit for comparing said displacement signal with a reference value corresponding to a desired displacement of said machine tool, said first feed means including a servo-controller operatively connected to said tool head and connected to said comparator for being controlled by said comparator to move said tool head, said first feed means also including a drive motor connected to said controller for effecting movement of said tool head, a tachometer connected to said drive motor for measuring rotations of said drive motor, said tachometer being connected to said controller for feeding a signal back to said controller for use with a signal from the comparator. 
     
     
       12. A device according to claim 11, wherein said second feed means comprises a feed mechanism connected between said tool head and said machine tool for repositioning said tool head to a zero reference position from which said displacement is measured by said displacement pickup.